There is a growing demand for multifunctional devices that are capable of transmitting and/or receiving wireless signals for a number of different applications operating over a number of different frequency bands. For example, mobile devices are often required to operate in a number of countries, each employing different communication frequencies and standards. Furthermore, the device may require access to multiple wireless services such as penta-band cellular services, GPS, Bluetooth, WiFi, DVB-H, UWB, AM/FM/DAB radio reception and wireless internet access. Traditionally, this means that a number of different antennas are required with corresponding circuitry and this has significant implications on the overall dimensions of the device, its shape and industrial design—these features being of considerable importance to an end user.
Several Cognitive Radio (CR) system architectures have been proposed which may help to overcome some of these challenges. In particular, Spectrum Sensing Cognitive Radio (SSCR) has been proposed with the aim of providing an improved and more reliable service by making more efficient use of the frequency spectrum. It is envisaged that a CR device would change its communication frequency whenever necessary—for example, to avoid interference and spectrum “traffic jams” or when more bandwidth is needed such as to send a video clip. It has therefore been proposed that a CR device would be configured to operate in the following two modes:                A ‘Listening’ mode, where the radio monitors the airspace for available spectrums/channels—an Ultra Wide-Band (UWB) antenna has been proposed for performing this listening/sensing function; and        An ‘Application’ mode, where the service requested by an application determines the frequency or bandwidth requirements of the device—for example, in current mobile communication systems, a high data rate service such as video call may be routed via High Speed Downlink Packet Access (HSDPA) using several channels. Thus, at least one frequency reconfigurable narrowband antenna will likely be required for performing the application function.        
However, as above, the space available for these antennas and their supporting circuitry will be limited in a portable CR device.
It will be understood that the term Ultra Wide-Band (UWB) is used throughout to denote a relatively large frequency range and is not limited to a specific range of frequencies such as those defined as UWB by the US Federal Communications Commission (FCC).
From the above, it will be apparent that tuneable antenna technology is a key requirement for an effective CR device as well as an enabling technology for advances in other mobile devices. Tuneable antennas will not only save space but will also enable devices to sense a user's interaction, environmental conditions and network requirements, and to reconfigure the antenna accordingly to maximise radiation performance. However, in conventional designs, it has been found that an antenna's frequency tuning range is often limited due to its physical dimensions.
It is therefore an aim of the present invention to provide a reconfigurable antenna which helps to address the above-mentioned problems.